Many devices are known in the prior art for monitoring a remote location and generating an alarm signal at the remote location in response to detection of a predetermined condition.
For example, U.S. Pat. No. 4,668,941 discloses a method and apparatus for discriminating sounds due to the breakage of glass and then triggering an alarm system. The sound of breaking glass comprises a low frequency or thump sound at the moment of breakage, followed by a tinkle sound caused by collision of the glass fragments, this sound being of lower amplitude. The apparatus disclosed in U.S. Pat. No. 4,668,941 relies on the discovery that irrespective of the size and shape of the glass and the characteristics of the surroundings, the thump sound has substantial low frequency components and the tinkle sound has substantial high frequency components. The method lies in identifying the high and low frequency components in that order and separated by a short time interval.
The apparatus disclosed in U.S. Pat. No. 4,668,941 comprises a microphone feeding into a high frequency and a low frequency channel. The low frequency channel comprises a low frequency bandpass filter followed by an amplifier and a voltage comparator, which provides an output voltage when the amplitude of the signal from the bandpass filter and amplifier exceeds a preset threshold value. The signal from the comparator is applied to a monostable multivibrator, which triggers a timer after a predetermined delay. The timer produces a signal of predetermined duration, thereby establishing a time window which enables an AND gate. The signal from the microphone is also applied to a high frequency channel comprising a high frequency bandpass filter and a voltage comparator, the output of which is applied to the other input of the AND gate.
Thus in accordance with U.S. Pat. No. 4,668,941, the AND gate outputs an alarm trigger signal in response to the detection of a predetermined high frequency signal having an amplitude above a predetermined threshold within a predetermined time after detection of a predetermined low frequency signal having an amplitude above another predetermined threshold.
A similar system for detecting glass breakage is disclosed in U.S. Pat. No. 4,134,109. This system utilizes transducers to convert acoustic waves received into electrical signals then analyzes the signal strength (amplitude), frequency content and the pattern of the signal and signal intervals to discriminate the sound of glass breakage from background or spurious noise.
The foregoing prior art relating to systems for detecting glass breakage suffer from the disadvantage that the rate of false alarms is inherently high. In the first place, because the frequencies of the thump and tinkle sounds of glass breakage will vary over a wide range for different types of glass of different dimensions, the bandpass filters of the detection system must have a wide passband. The wider the passband, the greater the likelihood that the background or spurious noise will include a frequency component within that passband. Thus, because more background or spurious noise will be detected, the probability of noise signals which mimick the sound of glass breakage within the range of the passband is correspondingly high.
Secondly, such detection systems are designed to detect a single sequence of events, that is, the thump sound followed within a predetermined time interval by a tinkle sound. If the background or spurious noise mimicks the sounds of this single sequence of events, then a false alarm will be triggered. Common sense dictates that the greater the number of events in the sequence which must be detected before an alarm signal is triggered, the lower the risk that a false alarm will be triggered.
Thirdly, although the foregoing systems detect the duration of an interval between two signals falling within a predetermined frequency passband, they do not detect the duration of the signals themselves. This again increases the probability of a false alarm because the signals being detected are not limited as to duration. The probability that the background or spurious noise will include a signal of first frequency followed by a signal of second frequency within a predetermined time interval is far greater than the probability that the background or spurious noise will include a signal of first frequency and first duration followed by a signal of second frequency and second duration within a predetermined time interval. In other words, if more characteristics of the received signal are detected, then more spurious signals which lack any one of those characteristics can be filtered out, thereby reducing the probability that a false alarm will be triggered.
Another material prior art reference is U.S. Pat. No. 4,558,181, which discloses a portable, self-contained device for monitoring a selected local area for the occurrence of any one of a plurality of preselected conditions. The monitoring device can be attached to any telephone jack. The standard telephone receiver set is then plugged into the monitoring device. After connection to an electrical outlet, the monitoring device is ready for operation by internal circuits which allow the monitoring device to create complex messages which are sent over the telephone lines in response to any one of a plurality of predetermined alarm conditions.
In particular, the device includes a monitoring device having an alarm condition responsive to a non-connected sound alarm such as a smoke detector or a burglar alarm, which alarm operates adjacent the monitoring device for a predetermined time duration indicative of an alarm condition. The detailed circuitry for providing a telephone alarm signal in response to sound recognition of local alarm signal is shown in FIGS. 5 and 13 of U.S. Pat. No. 4,558,181. A more general depiction of this detection circuitry is shown in FIG. 1 herein.
As shown in FIG. 1, the system for detecting a smoke alarm in accordance with U.S. Pat. No. 4,558,181 comprises a microphone 2 for transducing received acoustic signals into electrical signals. The signal output from the microphone 2 is amplified by amplifier 4. When switch 6 is closed in response to a control signal from the microprocessor (not shown), the amplified signal is applied to the high pass filter 8, which is used to charge capacitor 10 which contains a charge representing the average level voltage detectable by the analog value on output line 12. The analog value of the voltage on line 12 is periodically read by an analog-to-digital converter (not shown). If this value exceeds a certain level, it is indicative of a detection or alarm condition. The existence of the alarm condition selects a particular word or phrase from a voice processor (not shown), which message is sent over the telephone line. The detection system disclosed in U.S. Pat. No. 4,558,181 operates on the principle of detecting whether the filtered ambient acoustic signal exceeds a predetermined amplitude threshold for a preselected duration. Thus, it suffers from the second and third disadvantages of the prior art relating to glass breakage detection already discussed above. Since this detection system responds to a single occurrence of a signal having a frequency within a predetermined range and an amplitude in excess of a predetermined threshold, the risk that a spurious signal falling within this class of signals is greater than would be the case if the parameters of the recognized signal were more narrowly defined.